Liquids and viscoelastic material repellent and anti-biofouling coatings

ABSTRACT

A coated smooth surface of a substrate repels liquids and viscoelastic materials and can have anti-staining and anti-biofouling properties. The smooth surface can be coated by applying a chemical layer on the surface and a lubricant. Such coated surfaces are useful for use in toilets, urinals, or other devices for the processing of liquids and viscoelastic materials such as solid or semi-solid metabolic waste of human digestive system. Such coated surfaces can also be applied to windows for buildings or vehicles such as automobiles or camera lenses to repel liquids (e.g. rain), ice, frost, insect residues, and birds&#39; feces.

CROSS REFERENCE TO RELATED PARAGRAPH

This application is a continuation of U.S. application Ser. No.17/202,512, which is a continuation of U.S. application Ser. No.16/113,342 filed Aug. 27, 2018, which is a continuation of InternationalApplication No. PCT/US2017/062206 filed Nov. 17, 2017, which claims thebenefit of U.S. Provisional Application No. 62/424,062 filed Nov. 18,2016, the entire disclosures of which are hereby incorporated byreference herein.

TECHNICAL FIELD

The present disclosure relates to a repellant coating on a smoothsurface that can repel both liquids and viscoelastic solids, withapplications for anti-fouling, anti-staining, anti-scaling, andwater-saving systems such as toilets and urinals or other applicationsfor repellant coatings. The repellant coating includes a chemical layeron the smooth surface and a lubricant thereon.

BACKGROUND

Water shortage is a serious global problem. In areas, such as Africa andIndia, water is in very high demand for living and agriculture. Numerousresearchers have been focused on desalination of brine to generate morewater, and on advancing piping system in agriculture to save water.However, much less attention has been focused on water usage forflushing toilets. Specifically, two-thirds of people in the world haveaccess to flushing toilet. See World Health Organization and UNICEFJoint Monitoring Programme. Progress on Drinking Water and Sanitation,2015 Update and MDG Assessment. Each of them flush a toilet at afrequency 5 times per day on average (around 8 gallon of water), and 30billion gallon of water is flushed away globally per day. Certainregions such as Brazil make use of rainwater to flush toilets. However,rainwater could cause a number of health problems, especially bacterialinfection.

Articles have been developed with modified surfaces for fluid and solidrepellency. See, e.g., U.S. Patent Publication No. 2014-0342954; U.S.Pat. No. 4,844,986 and Wang et al. Covalently Attached Liquids: InstantOmniphobic Surfaces with Unprecedented Repellency. Angewandte ChemieInternational Edition 55, 244-248 (2016).

However, engineered surfaces that are non-sticky to both semi-solid(e.g. feces, wet molding clay, mustard, etc.) and bacteria are notwell-studied and developed because these visco-elastic semi-solids andbacteria are sticky to most surfaces. Existing slippery surfaces,including superhydrophobic surfaces and slippery liquid-infused poroussurfaces (SLIPS), are either sticky to bacteria (i.e., superhydrophobicsurfaces), or not specifically designed for sticky issues ofviscoelastic solids (i.e., superhydrophobic surfaces and slipperyliquid-infused porous surfaces). See Wong, T.-S. et al. Bioinspiredself-repairing slippery surfaces with pressure-stable omniphobicity,Nature 477, 443-447, 2011.

Superhydrophobic surfaces are slippery to water, or even oils usingspecial surface structure designs. However, these surfaces typicallylose its repellency under the exposure of biological liquids, such asblood and bacteria fluids. For bacterial fluids, biofilm can form ontothe surface structures and destroy the superhydrophobic property. Inaddition, superhydrophobic surfaces and slippery liquid-infused poroussurfaces are not known to repel viscoelastic semi-solids, such as fecesor mud.

Accordingly, there is a need for new surface technology that provides asimple solution to repel both liquids and viscoelastic solids forextreme water-saving applications or easy-to-clean medical devices.

SUMMARY OF THE DISCLOSURE

Advantages of the present disclosure include a surface design that canrepel both liquids and viscoelastic semi-solids and solids (e.g.,viscoelastic materials). Such surfaces are useful for anti-fouling,anti-staining, anti-scaling, and extreme water-saving applications oreasy-to-clean medical devices or other applications that benefit from arepellant coating. The surface features a chemical layer and lubricatingfluid thereon. Such a coating system is advantageously over a relativelysmooth surface resulting in a corresponding smooth lubricating layerthat repels both liquids and viscoelastic materials. This lubricatingsystem can provide minimal surface area and hence minimized adhesion toany impacting viscoelastic material.

These and other advantages are satisfied, at least in part, by a coatingon a smooth surface of a substrate wherein the coating comprises achemical layer on the surface of the substrate and a lubricant layerover the chemical layer. Advantageously, the surface of the substrate tobe coated is relatively smooth, e.g., the surface has an averageroughness R_(a) of less than about 4 μm, e.g., less than about 2 μm andless than about 1 μm average surface roughness and even less than about500 nm, e.g., less than about 100 nm. An advantage of the coating of thepresent disclosure is that the underlying surface substrate is notroughened prior to depositing the coating on the surface.

In an aspect of the present disclosure, the coated surface is part of asurface of a toilet, e.g., an inner surface of a toilet, a urinal, orother device that is useful for processing liquids and viscoelasticmaterials such as human waste. The minimized adhesion between theviscoelastic materials and the coated surface advantageously allowminimal water consumption to remove the materials from the devices. Thesmooth surface interface and the liquid-repellent function can alsoprevent buildup of mineral deposits (e.g., calcium and magnesiumcarbonates) and other materials that cause stains such as caused by hardwater. Such coated surfaces can also be applied to windows for buildingsor vehicles such as automobiles or camera lenses to repel liquids (e.g.,rain), ice, frost, insect residue, and bird feces. Furthermore, suchcoated surfaces can delay frost or ice formation and can significantlyreduce de-icing time. In an embodiment of the present disclosure, awindow made of glass comprises the coating on a smooth surface thereof.The chemical layer and lubricant on such a glass surface can have arefractive index that matches the glass, e.g., the chemical layer andlubricant have a refractive index of between about 1.3 to about 1.6,e.g., between about 1.4 to about 1.5.

In practicing certain aspects of the present disclosure, water or otheraqueous fluids can be applied to a device having a coating of thepresent disclosure to remove a viscoelastic material or a liquiddeposited on the coating. Water or other aqueous fluids can be appliedto the viscoelastic material or liquid on the coating to remove thematerial or liquid from the coating. The process advantageously canapply a minimal amount of aqueous fluids or other fluids to theviscoelastic material on the surface of the substrate to remove theviscoelastic material therefrom. In addition, lubricant, either the sameor a different lubricant than used to prepare the repellant coating, canbe reapplied to the chemical layer to renew the coating system on thesurface of the substrate.

Another aspect of the present disclosure includes a process forpreparing a repellant surface. The process comprises applying a coatingcomposition onto the surface to form a chemical layer on the surface. Alubricant can then be applied to the formed chemical layer.Advantageously, the lubricant layer can be applied to form a stablelubricant layer over the chemical layer. The method can advantageouslybe applied to a smooth surface of a substrate. Substrates that can beused with the coating system of the present disclosure include, forexample, metals, ceramics, glasses, or any combination thereof.

Embodiments of the present disclosure include one or more of thefollowing features individually or combined. For example, the coatingcomposition can include a (i) a polymerizable silane or siloxane orboth, (ii) a solvent and (iii) a catalyst such as an acid catalyst. Inother embodiments, the chemical layer can be a polydimethylsiloxane. Instill further embodiments, the chemical layer can have nanometer heightand be formed from a silane or siloxane to produce a polymer anchored tothe surface such as a grafted polydimethylsiloxane. In otherembodiments, the lubricant can be one or more of an omniphobiclubricant, a hydrophobic lubricant, a plant-based lubricant and/or ahydrophilic lubricant such lubricants include a perfluorinated oil or asilicone oil or an olive oil or a hydroxy polydimethylsiloxane, forexample. In some embodiments, the lubricant or a different lubricant canbe reapplied to the chemical layer to renew the coating on the smoothsurface.

Additional advantages of the present invention will become readilyapparent to those skilled in this art from the following detaileddescription, wherein only the preferred embodiment of the invention isshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription are to be regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having thesame reference numeral designations represent similar elementsthroughout and wherein:

FIGS. 1 a, 1 b and 1 c illustrate schematically and with optical imagesadhesion of viscoelastic solids to different engineered surfacesincluding a superhydrophobic glass (FIG. 1 a ), a SlipperyLiquid-Infused Porous Surfaces (SLIPS)-coated glass (FIG. 1 b ), and aLiquid-Entrenched Smooth Surfaces (LESS)-coated glass (FIG. 1 c ). Thesuperhydrophobic glass was created using a commercially availablesuperhydrophobic coating (NeverWet, LLC). The SLIPS-coated glass had anunderlying surface roughness˜1 μm. The LESS-coated glass had anunderlying surface roughness of less than about 1 nm. Synthetic feceswith a solid content percentage of 30% (dynamic viscosity of about 2406Pa·s) were used in these experiments.

FIG. 2 illustrates a method to create a LESS coating onto varioussurfaces including glass, ceramic, and metals.

FIGS. 3 a and 3 b illustrate the fabrication process and roughness ofaccording to an embodiment of the present disclosure. FIG. 3 a describesa dip-in fabrication process (a spray-coating process can alternativelybe used). A smooth substrate was hydroxidized by oxygen plasma for 10min. Then the substrate was dipped in a coating solution for 10 s, anddried in air for 10 min. A lubricant layer was then applied onto thecoated substrate. FIG. 3 b shows the roughness of the first elementsmeasured by atomic force microscope (AFM), which is a grafted chemicallayer. The roughness is on a nanometer scale, indicating the smoothnessof the coated substrate.

FIGS. 4 a-4 b show the visco-elastic property of synthetic feces. FIG. 4a shows the storage and loss moduli (denoted as G′ and G″, respectively)of different artificial feces with different solid content fraction(e.g. 10%, 20%, 30%, 40%, 50%, and 60% of solid in synthetic feces).FIG. 4 b shows the phase change of different artificial feces underdifferent applied frequency.

FIG. 5 a shows the measurements of adhesion between synthetic feces andvarious surfaces. The work of debonding of each surface is normalized bythat of bare glass for the respective solid content. The standarddeviations of work of debonding were obtained from at least 4independent adhesion measurements.

FIG. 5 b shows the adhesion measurement of four different surfaces:glass, micro-roughened slippery liquid-infused porous surfaces(MR-SLIPS), nano-porous slippery liquid-infused porous surfaces(NP-SLIPS), liquid-entrenched smooth surfaces (LESS). Work of debondingwas measured with synthetic feces of 20% solid content percentage.Standard deviations of the work of debonding were obtained from at least5 independent measurements. The lubricant used for SLIPS and LESS wassilicone oil with a viscosity of 20 cSt.

FIG. 5 c shows the adhesion measurement of four different surfaces:glass, micro-roughened slippery liquid-infused porous surfaces(MR-SLIPS), nano-porous slippery liquid-infused porous surfaces(NP-SLIPS), liquid-entrenched smooth surfaces (LESS). Work of debondingwas measured with synthetic feces of 40% solid content percentage.Standard deviations of the work of debonding were obtained from at least5 independent measurements. The lubricant used for SLIPS and LESS wassilicone oil with a viscosity of 20 cSt.

FIG. 5 d shows the adhesion measurement of four different surfaces:glass, micro-roughened slippery liquid-infused porous surfaces(MR-SLIPS), nano-porous slippery liquid-infused porous surfaces(NP-SLIPS), liquid-entrenched smooth surfaces (LESS). Work of debondingwas measured with synthetic feces of 60% solid content percentage.Standard deviations of the work of debonding were obtained from at least5 independent measurements. The lubricant used for SLIPS and LESS wassilicone oil with a viscosity of 20 cSt.

FIG. 6 is a plot comparing the work of debonding of synthetic feces ofvarying solid contents of surfaces with different roughness, includingmicro-roughened slippery liquid-infused porous surfaces (MR-SLIPS),nano-porous slippery liquid-infused porous surfaces (NP-SLIPS),liquid-entrenched smooth surfaces (LESS). The data is normalized by thework of debonding of 40% solid content synthetic feces on LESS.

FIG. 7 compares adhesion of a viscoelastic solid to uncoated glass,superhydrophobic glass (SHS), SLIPS-coated glass, and LESS-coated glass.The superhydrophobic glass was created using a commercially availablesuperhydrophobic coating (NeverWet, LLC). The underlying substrate ofthe SLIPS-coated glass has a surface roughness˜1 μm. The LESS-coatedglass had an underlying surface roughness of less than about 1 nm. Thesolid content percentage of the synthetic feces used is 30%. Dyed water(in blue) is sprayed onto the surface for cleaning purpose.

FIGS. 8 a and 8 b illustrate a human feces dropping test on differentcommercially available surfaces compared to a LESS-coated glass surface.FIG. 8 a schematic shows a human feces dropping test procedures, whichincludes: I) feces dropping from a height of 75 mm, II) feces impactingonto the test surfaces, and III) releasing the surface from horizontalto vertical to determine if feces will adhere onto the surface or not.FIG. 8 b shows optical images of test results for different surfaces.The human feces adhere onto ceramic, Teflon, and silicone, but slide offfrom the LESS-coated glass.

FIG. 9 shows the results of a human feces dropping test on SLIPS-coatedaluminum. The SLIPS-coated aluminum had an underlying surfaceroughness˜1 μm. The lubricant used here is Krytox 101 (DuPont,viscosity˜18 cSt), whose viscosity is similar to the silicone oil(viscosity 20 cSt) used on LESS. The human feces (˜10 grams) weredropped from ˜80 mm height. After 1-3 feces drops, the SLIPS-coatedaluminum included feces residue.

FIG. 10 is a plot comparing anti-bacterial performance of variouscoatings. The test included bacteria adhesion test with two types ofbacteria found in rainwater on glass, SLIPS with micro-roughened glasssubstrate (MR-SLIPS), SLIPS with nano-roughened glass substrate(NR-SLIPS), and a LESS-coated glass. For MR-SLIPS, NP-SLIPS, and LESSthere were no bacteria colony detected on these surfaces.

FIG. 11 is a plot comparing anti-bacterial performance comparison. Thetest included bacteria adhesion test with Escherichia coli contaminatedsynthetic urine on glass, SLIPS with micro-roughened glass substrate(MR-SLIPS), SLIPS with nano-roughened glass substrate (NR-SLIPS), andLESS-coated glass. For MR-SLIPS, NP-SLIPS, and LESS there were nobacteria colony detected on these surfaces.

FIGS. 12 a and 12 b illustrate a sterilization test on PDMS-graftedsurfaces with different surface roughness. FIG. 12 a illustratesexperimental procedures for testing sterilization on these surfaces. Allsurfaces were contaminated with E. coli biofilm, and then sterilizedwith bleach and 70% of alcohol for 10 min before lubrication withsterilized silicone oil. Afterwards, all surfaces were lubricated andincubated with agar film for 36 hours. FIG. 12 b is a plot comparingwhether bacterial colonies would grow on the surfaces. As shown in thefigure, bacterial colonies grow with underlying roughness (MR-SLIPS)while little or no bacteria are found on NP-SLIPS and LESS-coatedsurfaces. The inset image shows a SEM image of E. coli. This suggeststhat if these surfaces were contaminated (e.g., during an application ofinterest, if lubricant was depleted), LESS-coated surfaces could bereadily sterilized and their anti-biofouling function could be restored.

FIG. 13 shows the lubricant replenishment of LESS-coated surfaces. Theschematic shows the displacement wetting phenomenon on PDMS-graftedglass. The optical images show a facile lubricant replenishment process.

FIG. 14 is a plot which shows the durability of the lubricant layer(silicone oil with a viscosity of 20 cSt) under different flowconditions. Weight difference between LESS-coated and non-lubricatedsurface was measured and used to estimate the lubricant thickness.

FIG. 15 is another plot which shows the durability of the LESS coatingunder continuous feces impact-and-flushing cycles. Error bars representstandard deviations of three independent measurements.

FIG. 16 a illustrates water consumption required to remove syntheticfeces (10% solid content) from four different surfaces under differentflow rates. The flow rate ranges from 1 gallon per meter (gpm) to 2.5gpm. Four different surfaces include untreated glass, slipperyliquid-infused porous surfaces with micro-roughened surface morphology(MR-SLIPS), slippery liquid-infused porous surfaces with nano-poroussurface morphology (NP-SLIPS), and liquid-entrenched smooth surfaces(LESS).

FIG. 16 b illustrates the water consumption required to remove syntheticfeces (20% solid content) from four different surfaces under differentflow rates. The flow rate ranges from 1 gallon per meter (gpm) to 2.5gpm. Four different surfaces include untreated glass, slipperyliquid-infused porous surfaces with micro-roughened surface morphology(MR-SLIPS), slippery liquid-infused porous surfaces with nano-poroussurface morphology (NP-SLIPS), and liquid-entrenched smooth surfaces(LESS).

FIG. 16 c illustrates the water consumption required to remove syntheticfeces (30% solid content) from four different surfaces under differentflow rates. The flow rate ranges from 1 gallon per meter (gpm) to 2.5gpm. Four different surfaces include untreated glass, slipperyliquid-infused porous surfaces with micro-roughened surface morphology(MR-SLIPS), slippery liquid-infused porous surfaces with nano-poroussurface morphology (NP-SLIPS), and liquid-entrenched smooth surfaces(LESS).

FIG. 16 d illustrates the water consumption required to remove syntheticfeces (40% solid content) from four different surfaces under differentflow rates. The flow rate ranges from 1 gallon per meter (gpm) to 2.5gpm. Four different surfaces include untreated glass, slipperyliquid-infused porous surfaces with micro-roughened surface morphology(MR-SLIPS), slippery liquid-infused porous surfaces with nano-poroussurface morphology (NP-SLIPS), and liquid-entrenched smooth surfaces(LESS).

FIG. 16 e illustrates the water consumption required to remove syntheticfeces (50% solid content) from four different surfaces under differentflow rates. The flow rate ranges from 1 gallon per meter (gpm) to 2.5gpm. Four different surfaces include untreated glass, slipperyliquid-infused porous surfaces with micro-roughened surface morphology(MR-SLIPS), slippery liquid-infused porous surfaces with nano-poroussurface morphology (NP-SLIPS), and liquid-entrenched smooth surfaces(LESS).

FIG. 16 f illustrates the water consumption required to remove syntheticfeces (60% solid content) from four different surfaces under differentflow rates. The flow rate ranges from 1 gallon per meter (gpm) to 2.5gpm. Four different surfaces include untreated glass, slipperyliquid-infused porous surfaces with micro-roughened surface morphology(MR-SLIPS), slippery liquid-infused porous surfaces with nano-poroussurface morphology (NP-SLIPS), and liquid-entrenched smooth surfaces(LESS).

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure relates to a coated surface of a substrate thatincludes a chemical layer on the surface that can maintain a thinlubricant layer thereover to form a repellant surface. Advantageously,the surface is relatively smooth and does not require surface rougheningwhich contrast from certain other repellant surfaces such assuperhydrophobic surfaces and slippery liquid-infused porous surfaces.Compare FIGS. 1 a-1 b to FIG. 1 c . Coated surfaces in accordance withthe present disclosure are referred to herein as a liquid-entrenchedsmooth surface (LESS).

Coated surfaces in accordance with the present disclosure are useful oncertain devices such as toilets, urinals, or other devices for theprocessing of liquids and viscoelastic materials such as solid orsemi-solid metabolic waste of human digestive system. Such coatedsurfaces can also be applied to windows for buildings or vehicles suchas automobiles or camera lenses to repel liquids (e.g., rain), ice,frost, insect residues, and birds' feces. Furthermore, such coatedsurfaces can delay frost or ice formation and can significantly reducethe de-icing time.

In an aspect of the present disclosure, the surface of the substrate tobe coated is relatively smooth, e.g., the surface has an averageroughness at a microscale level, e.g., R_(a) less than a few microns, orless than a few hundreds of nanometers, and preferably less than a fewnanometers. In embodiments of the present disclosure, the surface of thesubstrate to be coated has an average surface roughness R_(a) less thanabout 4 μm, e.g., less than about 2 μm and less than about 1 μm averagesurface roughness. In certain other embodiments, the average surfaceroughness R_(a) is less than about 500 nm, e.g., less than about 100 nmand even less than about 10 nm. The average surface roughness R_(a) ofthe substrate to be coated can be in the range of greater than about 0.5nm to less than about 2 μm.

Average surface roughness can be measured by atomic force microscope(AFM) using tapping mode with a scanning area of 2×2 μm² for measuringaverage surface roughness in a 0.1-nanometer scale or equivalenttechnique. Average surface roughness can be measured by Zygo opticalprofilometer with an area of 475×475 μm² for measuring average surfaceroughness in a 1-nanometer scale or equivalent technique.

An advantage of the coatings of the present disclosure is that theunderlying surface substrate is not roughened as in a SLIPS coatingsystem. As such the coatings of the present disclosure can be readilyapplied on smooth surfaces without the need to affect the surfaceroughness. For example, the coatings of the present disclosure can bedirectly applied to toilet or urinal surfaces as well as window glass.Toilet and urinal surfaces generally have an average surface roughnessof less than about 500 and even as low as about 200 nm and 50 nm, andwindow glass can have an average surface roughness of less than about150 nm, e.g. between about 0.1 nm to about 100 nm. Alternatively, thesurface to be coated can be smoothened prior to applying the coatingsystem of the present disclosure. Hence, in certain embodiments, thesurface roughness does not need to be altered prior to applying acoating according to the present disclosure. In other embodiments, thesurface can be made to have a lower surface roughness, not roughened,prior to prior to applying a coating according to the presentdisclosure.

Substrates having a smooth surface that can be used in the presentdisclosure include those of silicon, ceramics, china, porcelain, glass,and metals such as carbon steel, copper, aluminum, and titanium. Ifmetal surfaces are chosen as the base substrate, oxygen plasma can beused to treat the surface to generate hydroxyl functional groupsthereon. The surface can be cleaned before applying the chemical layer.In an implementation of the present disclosure, a process for preparinga repellant surface includes applying a coating composition onto thesurface to form a chemical layer on the surface; and applying alubricant to the chemical layer.

A chemical layer can be formed on a smooth surface by applying a coatingcomposition onto the surface to form a chemical layer on the surface.The coating composition can be applied by a dip coating process orspraying. The chemical layer can also be formed by chemical vapordeposition (CVD) method. In an aspect of the present disclosure, thecoating composition includes three components: a coating chemical, asolvent, and a catalyst. Applying a coating composition onto a smoothsurface can form a chemical layer by anchoring the coating chemical tothe surface and, in certain embodiments, further polymerize the coatingchemical from the anchored coating chemical.

Useful coating chemicals include silanes and siloxanes such as, forexample, dimethyldimethoxysilane, trimethoxymethylsilane,1H,1H,2H,2H-perfluorododecyltrichlorosilane,1H,1H,2H,2H-perfluorodecyltrimethoxysilane,trimethoxy(3,3,3-trifluoropropyl)silane,dimethoxy-methyl(3,3,3-trifluoropropyl)silane,dimethoxy(methyl)octylsilane, trimethylmethoxysilane,diethoxydimethylsilane, dimethoxymethylvinylsilane,hexamethyldisiloxane, octyldimethylchlorosilane,octamethylcyclotetrasiloxane etc.,1H,1H,2H,2H-perfluorodecyltriethoxysilane,1H,1H,2H,2H-perfluorooctanephosphonic acid. Useful solvents includealcohols such as ethanol, isopropanol, ketones such as acetone,methylethylketone, chlorinated solvents such as chloroform, etc. Usefulcatalysts include acid catalysts such as sulfuric acid, hydrochloricacid, and superabsorbent polymers soluble for isopropanol or ethanoletc.

In one embodiment of the present disclosure, the chemical layer is asilane or siloxane polymer anchored onto the smooth surface, such as apolydimethylsiloxane grafted on the surface of the substrate, which canbe prepared from a coating composition including a polymerizable coatingchemical, a solvent, and a catalyst. As a demonstration,dimethyldimethoxysilane was used as a polymerizable coating chemical.The coating composition included 10 wt % of the dimethyldimethoxysilanein isopropanol and the catalyst made up 1 wt % of the coatingcomposition. In some embodiments, the chemical layer can have asub-nanometer height.

A lubricant that is compatible with the chemical layer is then appliedover the chemical layer. The lubricant can be applied by a wiping,spraying, etc. To form a stable lubrication layer, the lubricant shouldhave a strong affinity to the chemical layer or the substrate. In someembodiments, the lubricant can be one or more of an omniphobiclubricant, a hydrophobic lubricant and/or a hydrophilic lubricant suchlubricants include a perfluorinated oil or a silicone oil or a hydroxypolydimethylsiloxane (PDMS) or a plant oil. For example, perfluorinatedoils (e.g. Krytox oil) can form a stable lubrication layer on surfacesmodified by silanes and especially perfluorinated silanes. Silicone oilcan form a stable layer on surfaces having a chemical layer formed fromsiloxanes such as polydimethylsiloxane (PDMS) or grafted PDMS, forexample. Hydroxy PDMS can also form a stable layer on surfaces having achemical layer formed from siloxanes such as polydimethylsiloxane (PDMS)or grafted PDMS, for example. Mineral oils can form a stable layer onsurface having a chemical layer formed by alkyltrichlorosilanes oralkyltrimethoxysilanes with various chain lengths. Other lubricants thatwill be compatible with alkyltricholosilanes or alkyltrimethoxysilaneswith various chain lengths include alkane oils (e.g. decane, dodecane,hexadecane, or a mixture of them etc.), plant based oils such as oliveoil, palm oil, soybean oil, canola oil, rapeseed oil, corn oil, peanutoil, coconut oil, cottonseed oil, palm oil, safflower oil, sesame oil,sunflower oil, almond oil, cashew oil, hazelnut oil, macadamia oil,Mongongo nut oil, pecan oil, pine nut oil, walnut oil, grapefruit seedoil, lemon oil, orange oil, amaranth oil, apple seed oil, argan oil,avocado oil, babassu oil, ben oil, borneo tallow nut oil, cape chestnutoil, carob pod oil, coca butter, cocklebur oil, cohune oil, grape seedoil, Kapok seed oil, Kenaf seed oil, Lallemantia oil, Marula oil,Meadowfoam seed oil, mustard oil, Okra seed oil, papaya seed oil, Pequioil, poppyseed oil, pracaxi oil, prune kernel oil, quinoa oil, ramtiloil, Sapote oil, Shea butter, tea seed oil, tigernut oil, tomato seedoil, and other similar plant-based oils etc. The plant-based oils can beused alone or with other lubricants or as a mixture of plant-based oilsalone or with other lubricants. Lubricant viscosities ranging from ˜1cSt to ˜1000 cSt would be preferable at temperature range from 15° C. to25° C. Lubricant densities of less than about 1 g/cm³ would bepreferable at temperature range from 15° C. to 25° C.

In one aspect of the present disclosure, the coating according to thepresent disclosure is on a smooth surface, or part thereof, of a devicesuch as toilets, urinals, for the processing of liquids and viscoelasticmaterials such as solid or semi-solid metabolic waste of human digestivesystem or household or industrial sinks. Such coated surfaces can alsobe applied to windows for buildings or vehicles such as automobiles orcamera lenses to repel liquids (e.g., rain and fog), ice, frost, insectresidues, and birds' feces. Furthermore, such coated surfaces can delayfrost/ice formation and can significantly reduce the de-icing time.

In an embodiment of the present disclosure, a window made of glasscomprises the coating on a smooth surface thereof. The chemical layerand lubricant can have a refractive index that matches the glass, e.g.,the chemical layer and lubricant can have a refractive index of about1.3 to about 1.6, e.g., between about 1.4 to about 1.5, such as wherethe chemical layer is a silane and the lubricant has a refractive indexof about 1.3 to about 1.6, e.g., between about 1.4 to about 1.5.

In practicing certain aspects of the present disclosure, water or otheraqueous fluids can be applied to a device having a coating of thepresent disclosure to remove solid, e.g., viscoelastic material, orliquid on the coating. For example, viscoelastic materials, such asviscoelastic semi-solid waste, can be processed by contactingviscoelastic materials onto a coated surface of a substrate. The coatedsurface includes a chemical layer on the surface and lubricant layerthereover. Water or other aqueous fluids can be applied to theviscoelastic materials or liquids on the coating to remove the materialsfrom the coating. In addition, lubricant, either the same or a differentlubricant than used to prepare the repellant coating, can be reappliedto the chemical layer to renew the coating on the surface of thesubstrate.

In designing the coated substrates of the present disclosure, severalconsiderations should be given. The following provides particularconsiderations and examples for the design of a coating on a smoothsurface of a substrate in accordance with the present disclosure. Forexample, adhesion between a viscoelastic solid and the underlying solidsurface can be quantified by the adhesion energy at the solid-to-solidinterface. Specifically, the adhesion energy, G_(c), can be expressedas:

G _(c) =w _(a)Φ_(v)(da/dt,T,ϵ)  (1)

where Φ_(v) is a mechanical loss function, which depends on crack growthrate da/dt, temperature T, and strain c of the viscoelastic solid, andw_(a) is thermodynamic work of adhesion. Since the crack growth rate andstrain are inherit properties of the viscoelastic solid, reducing theadhesion of the viscoelastic solid and the substrate surface wouldrequire lowering the work of adhesion. Specifically, the work ofadhesion can be expressed as w_(a)=γ₁₃+γ₂₃−γ₁₂, which can be furthersimplified by Girifalco and Good equation as:

w _(a)=2(γ₁₃·γ₂₃)^(1/2)  (2)

where γ_(ij) is the interfacial energy at the i-j interface, and 1, 2,and 3 refer to the underlying solid substrate, the viscoelastic solid,and air, respectively. In order to reduce the work of adhesion, theadhesion between the underlying solid-air interface (γ₁₃) and theviscoelastic solid-air interface (γ₂₃) would need to be reduced.

Traditionally, there are two methods to decrease these interfacialenergies (γ₁₃ and γ₂₃). The first method involves surface chemistrymodification of the underlying solid substrate (e.g. silanization),which can effectively reduce γ₁₃. The second method involves lubricationbetween the viscoelastic solid and the substrate surface. As reportedpreviously, the lubricant could be absorbed by the viscoelastic solidsinstead of being adhered onto the substrate surface, resulting in thereduction of γ₂₃. In order to reduce both γ₁₃ and γ₂₃ concurrently, thelubricant would need to stably adhere on the underlying surface so as toretain a thin layer of lubricant. In this specific case, the adhesioninterface changes from a solid-to-solid interface (i.e., viscoelasticsolid-to-underlying solid substrate) to a solid-to-lubricant interface(i.e., lubricant-infused viscoelastic solid-to-lubricant-coated solidsubstrate).

In addition, the total work of adhesion between two surfaces is directlyproportional to their respective contact area, which could besignificantly increased by the presence of roughness. In 1960s, Carl A.Dahlquist showed experimentally that when the storage modulus of anadherent material is below a certain critical value (i.e., typically 0.3MPa), the material will begin to flow and form conformal contact withthe surface roughness of the adherent regardless of the appliedpressure. This is widely known as the Dahlquist criterion, which hasbeen the basis for the design of pressure sensitive adhesives. For aviscoelastic material that satisfies the Dahlquist criterion, anyroughness present on the adherent would further increase the surfaceadhesion. Therefore, reducing the surface roughness of the underlyingsolid substrate will be another important method to further reduce thesurface adhesion.

To verify the adhesion mechanics of different surfaces, we preparedsynthetic feces that have organic solid content very similar to that ofhuman feces for adhesion characterization on surfaces of varyingroughness (see Tables 1 and 2). The recipe of synthetic human feces wasdeveloped from the original recipe developed at the University ofKwaZulu Natal at South Africa. The synthetic human feces are composed ofyeast, psyllium, peanut oil, miso, polyethylene glycol, calciumphosphate, cellulose, and water. All solid components are expressed asdry mass, and the corresponding percentages are shown in Table 1. Thecomposition of the synthetic feces is biologically very similar to humanfeces (see Table 2). The viscosity of the synthetic feces can be tunedby the percentage of solid contents. We made synthetic feces with solidpercentage of 10%, 20%, 30%, 40%, 50%, and 60%. These synthetic feceswere used within 5 hours of preparation for viscoelasticitymeasurements, adhesion tests, and water consumption tests.

TABLE 1 Compositions of the synthetic feces. Ingredients % dry massNutrition Yeast 32.49 Biomass Psyllium 10.84 Fibre Peanut oil 17.31 FatMiso 10.84 Fibre/Protein/Fat Polyethylene glycol 12.14 CarbohydrateCalcium phosphate 10.84 Biomass Cellulose 5.53 Carbohydrate

TABLE 2 Comparison of dry mass percentage of human and synthetic feces.% dry mass - % dry mass - Organic Content Human feces Synthetic fecesBacterial Biomass 25-54  43.33 Protein or nitrogenous matter 2-25 6.21Carbohydrate 25 28.81 or undigested plant matter Fat 2-15 21.65

The work of adhesion, w_(a), at the feces and substrate interface can besimplified as w_(a)=2(γ₁₃·γ₂₃)^(1/2) by the Girifalco and Good equation.Health human feces contain about 70% of water with the rest consistingof organic matters. Therefore, we can estimate the upper bound of theirsurface energy (γ₁₃) to be similar to that of water (i.e., γ₁₃≤˜72mJ/m²). From previous literature, glass surface (γ₂₃) has surface energyas 310 mJ/m². Based on the aforementioned equation, the work of adhesionbetween human feces and glass surface (w_(a0)) is ≤299 mJ/m².

There are three different methods to reduce the work of adhesion. Thefirst method involves silanization of the glass surface withgrafted-polydimethylsiloxane (PDMS). Since the grafted PDMS has nearlyidentical chemical structure as the silicone oil, one can assume theirsurface energies to be similar. The surface energy of silicone oil ismeasured to be about 21 mJ/m². Therefore, the work of adhesion betweenthe synthetic feces and a PDMS-grafted glass (w_(a1)) would be ˜78mJ/m². In the second method, a lubricating layer (silicone oil) can beadded in between the feces and an untreated glass surface to reduce theadhesion. Since silicone oil is preferably to be absorbed to feces, onlythe surface energy of feces (γ₁₃) changes. Note that the silicone oilcould be partially or fully infused into the feces, therefore we assumethat the surface energy of feces (γ₁₃) to be ˜72 mJ/m²≥γ₁₃≥˜21 mJ/m². Asa result, the work of adhesion between the synthetic feces and asilicone-oil lubricated glass (w_(a2)) would be ˜299 mJ/m²≥w_(a2)≥˜161mJ/m². The third method involves coating a lubricating layer between thefeces and a chemically treated glass so as to retain a thermodynamicallystable lubricant layer on the substrate. In this case, we have ˜72mJ/m²≥γ₁₃≥˜21 mJ/m², and γ₂₃≈21 mJ/m². Therefore, the work of adhesionon the LESS-treated glass (w_(a3)) is ˜78 mJ/m²≥w_(a3)≥42 mJ/m².Overall, we have w_(a2)>w_(a1)≥w_(a3) and this trend is consistent withthe experimental measurements.

The solid contents of the synthetic feces range from 10 wt % to 60 wt %,which correspond to storage modulus of ˜1 Pa (˜10% solid content) to˜10⁵ Pa (˜60% solid content). These values closely emulate those of thefresh human feces. Based on the Dahlquist criterion, most of thesynthetic feces used here would be conformally contacting the roughsurfaces, particularly for the synthetic feces whose storage moduli aremuch less than 0.3 MPa. Our adhesion measurements showed that surfaceadhesion of the synthetic feces increases with the surface roughness(with average roughness, R_(a) ranges from 0.87±0.06 nm to 4.12±0.26μm), indicating a relatively smooth surface would be desirable inreducing surface adhesion with feces. Experimentally, it is found thatsurfaces with average roughness of less than 1 μm are preferable, oreven with a R_(a) of less than about 10 nm. (FIG. 1 c and Table 3).

TABLE 3 Average surface roughness of various base substrates. MaterialsCeramic Carbon Steel Titanium R_(a) (nm) 598 ± 35 451 ± 50 661 ± 30Note: R_(a) is measured based on an area of 478 μm by 478 μm.

In addition to the use of relatively smooth substrate, the lubricant hasto preferentially adhere to the substrate, which would require matchingsurface chemical affinity (i.e., forming a stable lubricant layer).Thermodynamically, to achieve this condition one should have,

ΔE ₁=γ_(B) cos θ_(B)−γ_(A) cos θ_(A)−γ_(AB)>0  (3)

where γ_(A) and γ_(B) are, respectively, the surface tensions of theforeign liquids to be repelled and the immiscible lubricant, γ_(AB) isthe interfacial tension at the liquid-liquid interface, and θ_(A) andθ_(B) are, respectively, the equilibrium contact angles of the foreignliquids and the lubricant on a given flat solid surface. In contrast toSlippery Liquid Infused Porous Surface (SLIPS) which requires a roughsurface for lubricant retention (see FIG. 1 b ), LESS use a relativelysmooth surface for adhesion reduction. (Typically a SLIPS structureincludes a synthetically roughened surface and has an average surfaceroughness greater than ˜150 nm and as high as 100 μm). In accordance tothe Dahlquist criterion, this smooth architecture is important forrepelling viscoelastic solids.

Since LESS employs a lubricant for its function against aqueous liquidsand viscoelastic solids, it will be desirable in certain embodiments todevelop a lubricant replenishment strategy to renew the surface in casethe lubricant layer is depleted over time due to evaporation andexternal fluid shear. During the lubricant replenishment process, thefollowing thermodynamic condition has to be satisfied so that it isenergetically favorable for the lubricant to displace the aqueousliquid-wetted surface,

ΔE ₂=γ_(B) cos θ_(B)−γ_(A) cos θ_(A)+γ_(A)−γ_(B)>0  (4)

In summary, LESS can be formed onto a surface if 1) the base substrateis relatively smooth, preferably with less than about 500 nm averageroughness, or even less than about 100 nm average roughness, and 2)ΔE₁>0 (stable lubricant formation), and 3) ΔE₂>0 (lubricantreplenishment).

LESS can be formed by first functionalizing the base substrate with adifferent surface chemistry, such that the lubricant can preferentiallywet onto the functionalized surface and can satisfy the criteria ΔE₁>0(stable lubricant formation) and ΔE₂>0 (lubricant replenishment).

In one of these approaches, the smooth substrate can be selected to behydrophilic with hydroxyl groups available on the surface (eithernaturally available or after specific surface treatment such as oxygenplasma). Examples of such surfaces include glass, silicon, ceramic,china, porcelain, and certain metals such as carbon steel, copper,aluminum, and titanium. If metal surfaces are chosen as the basesubstrate, oxygen plasma can be used to treat the surface to generatehydroxyl functional groups thereon.

Before applying the surface functionalization, the surface is preferablycleaned to remove any surface residues. One example for the cleaninginvolves the use of an alcohol, e.g., ethanol or isopropanol, anddeionized water to rinse the surface. After the surface is allowed todry, it can be treated with silane solution which enables alternation ofthe surface chemistry. Once the surface is clean, a solution containingsilane or siloxane molecules can be sprayed or wiped onto the surface,allowing these molecules to react with the hydroxyl groups and forming acovalently-bonded chemical layer on the substrate. See, e.g. FIG. 2 .The silane or siloxane solution can either be sprayed coated or wipedonto the surface and allowed to dry in ambient condition (temperaturefrom 0° C. to 60° C., and relative humidity from 30% to 80%) for 10 min.

One specific example of the silane solution includes 10 wt % ofdimethyldimethoxysilane with addition of 1 wt % concentrated sulfuricacid and was mixed successively with isopropanol as the solvent. Thesilanized substrate then becomes hydrophobic and can repel both waterand alkanes. The formation of the new surface functional group wasconfirmed by X-ray photoelectron spectroscopy (XPS) measurements showingthe formation of Si—O bonds associated with dimethyldimethoxysilane. Thethickness of the chemical coating layer can range from about 1 nm toabout 10 μm.

Once the chemical layer is formed, lubricant can be applied onto thefunctionalized surface either by wiping or spray coating. Examples ofthe lubricants include silicone oil or hydroxy polydimethylsiloxane.Lubricant viscosities ranging from ˜1 cSt to ˜500 cSt would bepreferable at temperature range from 15° C. to 25° C. In one of theexamples, the PDMS-grafted surface is preferentially wetted by siliconeoil, forming a stable lubricating layer. The rinsing process can beskipped as the excess coating chemical is soluble in lubricant (e.g.silicone oil). The thickness of the lubricant layer ranges from ˜1 nm to˜1 mm. Our experimental measurements have shown that this specificmaterial combination (i.e., silicone oil and PDMS-grafted surface,γ_(A)=71.1±0.2 mN/m, γ_(B)=20.7±0.3 mN/m, θ_(A)=106.5°±0.4°, θ_(B)=˜0°,γ_(AB)=31.7±0.1 mN/m) satisfies the relationship outlined in Eq. 3 (i.e,ΔE₁=9.2 mN/m). The two-step fabrication process generally takes lessthan 10 minutes.

As such, the coated surfaces of the present disclosure can be preparedby a facile fabrication process. FIG. 3 shows an embodiment of preparingsuch a coated surface. As shown, a covalently bonded chemical layer on asmooth substrate (FIG. 3 a ) can initially be formed. Forexperimentation, smooth glass slides were cleaned by ethanol and thenhydroxidized by oxygen plasma for 10 min before dipping them into acoating solution which formed a PDMS on the surface of the silds. Afterdipping into the coating solution for 10 s, the glass slides were driedin ambient condition for 10 min. After rinsing with toluene andisopropanol, the glass slides were coated to form a thin chemical layerwithout introducing any additional roughness (FIG. 3 b ). The rinsingprocess can be skipped as the excess coating chemical is soluble inlubricant (e.g. silicone oil here). Before lubrication, the coated glassslides repel alkane oils and water, and have the ability to self-clean.To complete the LESS, lubricant was applied onto the surface by spin orspray coating.

To form a stable lubricant layer, the substrate surface needs to have astrong chemical affinity to the lubricant. We used X-ray photoelectronspectroscopy (XPS) to show that dimethyldimethoxysilane as the adhesionlayer was successfully coated on quartz glass slides. A graft PDMS layerwas confirmed to be coated on the glass by the XPS data. The PDMS layerhas a strong affinity to silicone oil. After lubrication, the coatingcan repel water and complex aqueous liquid (e.g. sheep blood).

Different from the traditional SLIPS, the coated surfaces of the presentdisclosure, i.e., Liquid-Entrenched Smooth Surface (LESS), do notrequire surface roughness to maintain lubricant. The presence of thesurface roughness of SLIPS may lead to adhesion of the viscoelasticsolids upon impact. In certain embodiments, the surface roughness doesnot need to be altered prior to applying a coating according to thepresent disclosure. In other embodiments, the surface can be smoothened,not roughened, prior to prior to applying a coating according to thepresent disclosure.

To characterize the repellant performance of LESS-coated surface withother commonly-used and state-of-the-art materials, we used syntheticfeces with 20%, 40% and 60% solid content percentage to measure theadhesion on different surfaces. The viscoelastic property was measuredas shown in FIG. 4 . The control surfaces in these tests includeuncoated glass, glass lubricated with silicone oil, PDMS-grafted glass,and silicone-oil infused SLIPS samples with either microscale (R_(a)˜4μm) or nanoscale roughness (R_(a)<1 μm) on the underlying substrates. Tosimulate the condition of synthetic feces impact, we measured theaverage impact force by high speed camera (release height: 400 mm; fecesweight: 5 g; impact area: 24.5×25.3 mm²). Our measurements showed thatthe average impact forces were ˜0.23 N, ˜2.33 N, and ˜5.60 N for 20%,40%, and 60% synthetic feces, respectively. Therefore, we set 0.5 N, 5N, and 10 N as the upper limits of the loading forces in our adhesionmeasurement for 20%, 40%, and 60% synthetic feces, respectively.

Our results showed that lubrication on bare glass (without chemicalfunctionalization) can reduce the surface adhesion by ˜41% for syntheticfeces with 40% solid content (i.e., the stickiest sample in the test).In comparison, grafted-PDMS glass can reduce the surface adhesion by˜75%, while the LESS coating can reduce the adhesion by ˜90% (FIG. 5 ).Our measurements also showed that the adhesion increases with increasingunderlying surface roughness (FIG. 6 ). In general, the LESS-coatedsurface outperforms other control surfaces, including untreated surfaceswith or without lubrication, and SLIPS with different underlyingroughness (FIG. 7 ).

Additionally, we compared the adhesion performance of human feces onLESS and other state-of-the-art and commercially available surfaces.Specifically, we used glazed ceramic (a typical toilet material),Teflon, and silicone as the control surfaces (FIG. 8 ). For these tests,we used a setup that allows human feces samples to be released from adrop rig at the same height onto an acrylic support where the testcoating is placed. When the support pin for the acrylic surface isremoved, the surface drops from a horizontal position to a verticalposition, where the feces are expected to slide down the face of thesurface. In our tests, all the commercial surfaces show extremestickiness towards the human feces samples. LESS-coated glass, however,was the only surface showing non-stickiness towards the feces sample andleft no noticeable residue behind. Furthermore, we have shown in adifferent set of feces impact tests that traces of feces were left on aSLIPS-treated surface (with underlying surface roughness˜1 μm). See FIG.9 . Therefore, our tests further demonstrate that our LESS coatingoutperforms various state-of-the-art surfaces on repelling viscoelastichuman feces.

One important reason that urinals or toilets need to be flushed andcleaned regularly is to prevent the growth of bacteria, generation ofunpleasant odor, and spread of infectious diseases. In certain regions(e.g., Brazil), rainwater is used as the source for toilet flushing.However, rainwater can contain bacteria that may contaminate thesanitation facilities. Owing to the presence of the mobile lubricantinterface of LESS, we hypothesize that LESS may have comparableanti-biofouling performance as state-of-the-art materials. To verifythis, we performed biofouling analyses on LESS-coated substrate andother control surfaces using natural rainwater.

Specifically, we collected rainwater from a house roof in State College,Pa., USA, and measured its bacteria content and concentration. Therainwater was collected and then stored in a refrigerator at 4° C. Wetested the concentration of bacteria in rainwater by diluting therainwater by 10× with sterilized DI water, and spread 10 μL of therainwater and the diluted solution on a solid agar. After culturingthese samples in 37° C. incubator for 36 hours, we counted the number ofbacterial colonies on the agar surface to determine the concentrations.We identified the bacteria in the rainwater using a MALDI Biotypersystem; and these bacteria were identified as Staphylococcus aureus,Enterobacter cloacae, Escherichia vulneris, Escherichia hennannii,Acinetobacter calcoaceticus, Enterococcus mundtii (Table 4), which arecommonly found in rainwater.

TABLE 4 Bacteria identification from mass spectrometry. Organism Score(best match) Value Staphylococcus aureus 2.358 Enterobacter cloacae2.281 Escherichia vulneris 2.256 Escherichia hermannii 2.212Enterococcus mundtii 2.219 Acinetobacter calcoaceticus 2.077 Note: Anyscore value >2 indicates secure genus identification.

The detailed process of the bacteria identification is as follow. 50 μLof rainwater was spread uniformly onto four different agar mediums:Thermo Scientific™ Blood Agar (TSA with Sheep Blood) Medium, ThermoScientific™ MacConkey Agar Medium, BD BBL CHROMagar Orientation, and BDBBL MHB agar. Then the bacteria were incubated for 24 hours. Isolateswere prepared for analysis using a direct transfer method following astandard Bruker protocol. Individual colonies from 24-hour cultures weretransferred onto a MALDI target plate using a sterile pipette tip andallowed to dry. The cells were lysed by applying 1 μL of 80% formic acidsolution in water, samples were allowed to dry and 1 μL of 10 mg/mL HCCAmatrix solution in 50% aqueous acetonitrile containing 2.5%trifluoroacetic acid was applied to each sample and allowed to dry. Abacterial test standard (BTS; Bruker Daltonics) was used for instrumentcalibration and as a positive control. Matrix blank spots were includedin each analysis to ensure that the target plate was thoroughly cleanedand there is no carryover signal. MALDI mass spectra were acquired on aBruker Ultraflextreme MALDI TOF/TOF mass spectrometer in the linear,positive-ion mode. Spectra were processed using a factory defaultprocessing method for the Biotyper application and searched against aBruker library containing entries of 6903 cellular organisms using MALDIBiotyper Version 3.1 software (Bruker).

We rinsed the LESS-coated substrate, two SLIPS samples (one withunderlying microscale roughness and the other with nanoscale roughness),and uncoated bare glass with the collected rainwater for 1 min, and thenimmediately incubated the substrates by attaching solid agar onto thesurfaces in an incubator. After 36 hours of incubation, we counted thebacterial colonies on these surfaces. Specifically, no observablebacteria colonies were found on all SLIPS-coated and LESS-coatedsubstrates; whereas the untreated glass surfaces were contaminated withthe bacteria in the rainwater. Our results showed that theanti-biofouling performance of the LESS-coated substrate is comparableto existing state-of-the-art anti-biofouling materials (FIG. 10 ).

In addition to the rainwater tests, we further characterized thesesurfaces with a mixture of E. coli and synthetic urine. 10 mL of thiscontaminated urine was sprayed onto the test surfaces to simulate theurination process, followed by the aforementioned procedures for thebiofouling characterizations. Our test results are similar to thosefound in the rainwater tests, where all of the SLIPS and LESS-coatedsamples showed no observable bacteria colonies while the glass substrateshowed significant contamination with bacteria (FIG. 11 ). We havefurther shown that SLIPS and LESS-coated substrates can repel allbacteria-contaminated synthetic urine with a sliding angle of a droplet(10 μL) less than 5°. Furthermore, LESS exhibited strong repellencytowards both aqueous liquids (contact angle=105.5°±0.3° and contactangle hysteresis=0.8°±0.2°).

It is interesting to note that in the case where the lubricant is fullydepleted in SLIPS, bacteria biofilm can attach onto rougher substratespersistently. Even after sterilization with bleach and 70% of alcohol,there are still bacteria attached on the rough surfaces. In comparison,the biofilms that are initially attached onto the lubricant-depletedLESS-coated surface can be completely removed after the bleachsterilization. (FIG. 12 ). All of these results demonstrate that LESShas excellent anti-biofouling performance and, therefore, could minimizethe use of disinfectants or other aggressive chemicals currently usedfor cleaning and sterilization

Since LESS requires the use of lubricant for its function againstliquids and viscoelastic materials, it is desirable to reapply lubricantto renew the surface in case the coating is depleted over time such asrepeated toilet flushing. Since the PDMS-grafted substrate of LESS isdesigned to adhere the silicone oil as opposed to aqueous liquids, it ispossible to replenish the lubricant layer by incorporating small amountsof silicone oil in the flushing water so that the silicone oil canpreferentially wet the surface through displacement wetting (FIG. 13 ).Our experimental measurements have shown that Eq. (4) is satisfied(i.e., ΔE₂=91.3 mN/m>0) for the displacement wetting of the lubricant.Experimentally, we have further shown that silicone oil wets thePDMS-grafted ceramic surface even when the surface has been pre-wettedby water, and subsequently forms a functional layer to repel the water(FIG. 13 ).

We have investigated the durability of the LESS coating againstrealistic water flow (i.e., 1 gallon/min to 2.8 gallon/min) and impactof synthetic feces.

For the water flow test, we put LESS-coated glass slides into the flowsystem at a flow rate of 1 gallon/min and 2.8 gallon/min. Our setup iscapable of generating a flow rate from 1 gallon per minute (i.e., 3.8L/min with Reynolds number, Re˜4570, calculated based on the hydraulicdiameter) up to 2.8 gallon per minute (i.e., ˜10.6 L/min with Re˜13100).The estimated wall shear stresses generated by these flows range from0.093 Pa (at 1 gallon per min) up to 0.78 Pa (at 2.8 gallon per min),which are similar to those of typical toilets. We measured the weight ofthe glass slides and the glass slides with one slide lubrication. Beforeand after every 5 min of flushing, we measured the weight of LESS-coatedglass slides. With coated area (A) of 1875 mm² and lubricant density (ρ)of 0.95 g/mL, we can calculate the lubricant height (h) with weightdifference (W_(before)−W_(after)) by h=(W_(before)−W_(after))/ρ/A.

For the impact of synthetic feces test, we dropped ˜5 grams of synthetichuman feces from 400 mm height onto the surface at a tilting angle of45°. Then the LESS-coated surface was put into the flow system forcleaning at a flow rate of 1 gallon/min. We verified the completeremoval of the feces residues using fluorescent trace dye, which wasmixed with the synthetic feces during our tests. Before and after theimpact-and-flushing cycle, we measured the sliding angle of the surfaceusing a 10 μL water drop. The LESS-coated surface was considered to befully degraded if the sliding angle was >65°.

Our results show that the LESS coating can remain functional evenafter >100 continuous flushes of water at a flow rate of 2.8 gallon/min(FIG. 14 ), as well as ˜8 to 36 impact-and-flush cycles of syntheticfeces of various solid contents before further replenishment is required(FIG. 15 ). Compared to uncoated glass surfaces, LESS-coated surfacesreduce water consumption by up to 90% for various synthetic feces atdifferent solid contents. We have also conducted similarcharacterizations on SLIPS samples, and found that the water consumptionincreases with increasing underlying substrate roughness—a trend that isconsistent with the observations in the adhesion tests.

Our measurements also indicated that SLIPS with underlying surfacestructures and untreated glass could consume more flushing water thanliquid-entrenched smooth surfaces (LESS) to remove the feces residues.

Water flushing in real toilets can be simplistic modelled as an openchannel flow.

To simulate the condition of flashing a toilet, we designed an openchannel flow system, in which flow rate can be controlled. Aftermeasuring the flow height where the sample (i.e., artificial feces) islocated, we can calculate the hydraulic diameter

$\left( {D_{h} = {{4R_{h}} = \frac{4A}{P}}} \right),$

and then Reynolds number

$\left( {{Re}_{D} = \frac{\rho{vD}_{h}}{\mu}} \right).$

Flow Rate 1.0 1.5 2.0 2.5 2.8 (gpm) Height 4.75 3.62 5.11 4.27 4.15 (mm)Re 4578 7136 9049 11632 13081

As in all flow rates, the Reynolds number is larger than 4000, the flowis in turbulence. And the wall shear is calculated as follow:

Flow Rate (gpm) 1.0 1.5 2.0 2.5 2.8 Wall Shear (Pa) 0.093 0.33 0.28 0.600.78

The wall shear in our open channel flushing system is in the same orderof magnitude with the shear in real toilet. Therefore, the open channelflushing system can realistically simulate the real toilet flushingcondition.

With this flushing system, four flow rates were chosen in the tests,including 1.0 gpm, 1.5 gpm, 2.0 gpm, and 2.5 gpm. Four differentsurfaces were tested (glass, micro-roughened slippery liquid-infusedporous surfaces (IR-SLIPS), nano-porous slippery liquid-infused poroussurfaces (NP-SLIPS), LESS). Each surface was positioned with 45°tilting, and impacted by feces with different solid content (10% to 60%)at a height of 400 mm. The synthetic feces were mixed with UV powders(0.1 wt % in solid content) to enhance the visibility of feces residueson surfaces. Under different flow rates, we counted the time requiredfor the surfaces to be completely free of any residues, and thencalculated the corresponding amount of water required.

FIGS. 16 a and 16 b show the water consumption required to remove feceswith 10% and 20% solid content from the surfaces. liquid-entrenchedsmooth surfaces (LESS) require the least amount of water, whileuntreated glass takes the largest amount of water. MR-SLIPS requiresimilar amount of water to glass for two reasons. First, the feces with10% and 20% solid content show more viscous property, which means theybehave like viscous fluid. This viscous fluid took away large amount oflubricant on MR-SLIPS, and therefore more feces residues can adhere ontothe surface. Second, the viscous feces removed large amount of lubricantand exposed the underlying surface structure. Once the feces residuesattached on the surface structure, they are much harder to be removed.

It is noticeable that water consumption is ˜10 times less in 10% solidcontent than 20% solid content. The feces with 20% solid content aremuch more elastic than 10% ones, while still behave like viscous flow.This property will cause stronger adhesion of feces residues onto thesurfaces, which leads to more water consumption to remove the residues.With the increase in elasticity of the feces to 40% solid content, thewater consumption increases.

The synthetic feces were repelled from the slippery surfaces (MR-SLIPS,NP-SLIPS, and LESS), while sticking to glass surfaces when the solidcontent of the synthetic feces increased to 30% and 40%. Differentamount of residues were left on slippery surfaces due to differentsurface roughness of underlying substrates. Compared to all of thesesurfaces, glass surface required the largest amount of water to removethe residue. Water consumption increased with underlying surfaceroughness in liquid-infused slippery surfaces. In general, the waterconsumption required to clean the synthetic feces (30%-40% solidcontent) on the LESS is only ˜10% to that of the glass (FIG. 16 c andFIG. 16 d ).

Synthetic feces with 50% and 60% solid content requires ˜½ to ˜⅓ ofwater consumption required to remove synthetic feces with 40% solidcontent. Synthetic feces with higher solid content are less adhesivethan those with 30% and 40% solid content.

Similar to the situation with the artificial feces with 40% solidcontent, the feces bounced off the slippery surfaces (MR-SLIPS,NP-SLIPS, and LESS), while was partially sticking to glass surfaces whenthe solid content increased to 50% and 60%. In general, the waterconsumption required to clean the artificial feces (50%-60% solidcontent) on the LESS is only ˜5% to that of the glass (FIG. 16 e andFIG. 16 f ).

There are seven types of stool or feces defined by Bristol stool scale(1. separate hard lumps, like nuts (hard to pass); 2. sausage-shaped butlumpy; 3. like a sausage but with cracks on the surface; 4. like asausage or snake, smooth and soft; 5. soft blobs with clear-cut edges;6. fluffy pieces with ragged edges, a mushy stool; 7. watery, no solidpieces). The synthetic feces with 10% to 60% solid content almost coverall seven types of stool. Among seven types of stool or feces, type 4and 5 would be most healthy and common stool; while type 6 and 7probably are diarrhea. Meanwhile, type 4 and 5 would require higherwater consumption to clean toilets. We estimate that LESS only need <20%of water consumption of traditional toilet surfaces to maintain a cleansurface.

Toilets/Urinals

One example to apply LESS coating on a toilet bowl/urinal involvessurface cleaning, surface functionalization and lubricant coating.Surface cleaning involves the use of ethanol wipe to clean the toiletbowl/urinal. If the toilet bowel/urinal has been used and staining isobserved, it is recommended to use commercially available acid cleanerto remove the stain before the surface functionalization step. Once thesurface is cleaned and left dried, then surface functionalization can beproceeded which involves spraying or wiping of a silane solution (e.g.,composed of isopropanol alcohol, dimethyldimethoxysilane, and an acidcatalyst such as sulfuric acid) onto the toilet/urinal surface to form apermanent functional layer. Once dried, the surface can be sprayed orwiped with a matching lubricant, such as silicone oil (a viscosity of 20cSt, Sigma Aldrich, CAS No. 63148-62-9), to complete the LESS coatingprocess. The lubricant can be reapplied or replenished from an externalsource (e.g., a lubricant reservoir) as needed. A LESS-coated toiletthus prepared repels human urine and greatly reduces stickiness of humanfeces.

Windows

In addition to repel human waste, LESS can also be applied onto a windowor camera lens to repel liquids (e.g., rain), ice, frost, insectresidues, and birds' feces. As an application example, we have appliedLESS coating on a car windshield (Model: Mazda CX-5, 2013). To beginwith, the windshield was first cleaned with an ethanol clean wipe toremove residues on the surface. Then, a previously described silanesolution (composed of isopropanol alcohol, dimethyldimethoxysilane, andan acid catalyst such as sulfuric acid) was either sprayed or wiped ontothe windshield and left for complete drying for ˜20 minutes. Once dried,this solution will form a permanent hydrophobic coating onto thewindshield. Then silicone oil was sprayed or wiped onto the windshieldto complete the coating process. This lubricant layer can be reappliedor replenished from an external source (e.g., a windshield wiper fluidreservoir) as necessary. In this specific example, the coating wasapplied under environment conditions at 4° C. and a relative humidity of˜60%.

To maintain the optical clarity of the window or lens, the lubricantused for the LESS coating will need to have a similar refractive indexas the base glass substrate. For example, typical glass has a refractiveindex, n˜1.4-1.5 and a typical matching lubricant would have arefractive index that is within the difference of +/−0.1. Examples ofthe lubricants include silicone oil (n˜1.4) and plant oils(n˜1.46-1.47).

The LESS-coated windshield has superior performance in repelling raindroplets as compared to an uncoated windshield glass and a windshieldglass treated with commercial available coatings (e.g., Rain-X).Specifically, when the windshield is in static position, water dropletsof ˜1-3 mm will begin to slide off from the windshield. When the car istravelling at a speed of ˜20-25 mph, water droplets of ˜1-2 mm begin toshed off from the windshield. At a speed of ˜30-35 mph, most of thewater droplets (with size<˜1 mm) will shed off from the windshield. At aspeed over ˜45 mph, all of the water droplets (with size<˜1 mm) will beshed off immediately when they impact the glass. In comparison, raindrops (with size>˜1 cm) remain stuck to the uncoated windshield evenwhen the speed reaches ˜40 mph, and water droplets of >˜5 mm begin toshed off at >˜30 mph on Rain-X-coated windshield according to thepublished demonstration.

Anti-Frosting/Icing

Furthermore, LESS-treated surface can delay frost/ice formation and cansignificantly reduce the de-icing time. As an example, LESS-coatedsilicon can delay the ice formation as compared to uncoated smoothsilicon. Specifically, ice covers a 25 mm-by-25 mm area by ˜9.7±0.4 minon untreated silicon wafer, and by 13.7±1.1 min on LESS-coated silicon.Meanwhile, the de-icing time of LESS-coated silicon is 1.3±0.1 mincomparing to 1.5±0.1 min for untreated silicon.

The LESS-coated silicon was made through the two-step spray coatingprocess in room condition (23° C. and 50% relative humidity).Specifically, a smooth silicon wafer was cleaned with ethanol anddeionized water. A silane solution was then applied by spraying thesolution onto the cleaned, smooth silicon wafer. After the surface wasdried, lubricant (e.g. silicone oil) was sprayed on the surface tocomplete the LESS coating.

The anti-icing test was performed on a Peltier plate with a 90° tiltingangle. The surfaces were cooled down to ˜−5° C. at room environment (23°C. and 30% relative humidity). The icing and de-icing was recorded by acamera for measuring the time.

We conducted a field test to demonstrate the anti-frost performance ofLESS-coated windshield. The test was conducted in an early morningduring the winter time at State College, Pa. Specifically, thewindshield was covered with frost at −2° C. at 100% relative humidity onboth treated and untreated windshield. After wiping with the windshieldwiper twice, the LESS-coated surface was defrosted immediately andbecame visually clear, while the untreated area was still heavilycovered with frost with very low visibility. This indicates that frosthas a very low adhesion on the LESS-treated windshield as compared tothe untreated surface.

Examples

The following examples are intended to further illustrate certainpreferred embodiments of the invention and are not limiting in nature.Those skilled in the art will recognize, or be able to ascertain, usingno more than routine experimentation, numerous equivalents to thespecific substances and procedures described herein.

Fabrication Process of MR-SLIPS

Slippery liquid-infused porous surfaces with micro-roughened surfacestructure (MR-SLIPS) were fabricated by the following steps. First,laser roughening was applied on glass slides with 35% of maximum power(25 W) and 90% of maximum scanning speed by a laser cut machine(Universal Laser vls2.0). Second, the glass slides were furtherroughened by buffered oxide etch of HF (6:1) for 20 min. Third, thesurfaces were hydroxidized by oxygen plasma for 10 min, and then dippedinto the coating solution. After drying, silicone oil as lubricant wassprayed onto the surfaces.

Fabrication Process of NP-SLIPS

Slippery liquid-infused porous surfaces with nano-porous surfacestructure (NP-SLIPS) were fabricated by following steps. First, theglass slides were submerged into 1M of sodium bicarbonate aqueoussolution at 100° C. for 24 hours. Second, the surfaces were hydroxidizedby oxygen plasma for 10 min, and then dipped into the coating solution.After drying, silicone oil as lubricant was sprayed onto the surfaces.

Surface Roughness Measurement

Surface roughness of grafted-PDMS glass slide and nano-porous glassslide was measured by atomic force microscope (AFM) using tapping mode.The scanning area was 2×2 μm². Surface roughness of nano-porous glassslide and micro-roughened glass slide was measured by Zygo opticalprofilometer. The measured area was 475×475 μm². Measurement by AFMprovides roughness in 0.1-nanometer scale whereas optical profilometer(Zygo) provides roughness in 1-nanometer scale.

X-Ray Photoelectron Spectroscopy (XPS) Measurement

Untreated quartz glass slide and grafted-PDMS coated quartz glass slidewere characterized under XPS with area of 1 mm in diameter circle anddepth of 10 nm for 3 different spots.

Visco-Elasticity Measurement

Oscillatory rheology testing was conducted using a TA Instruments DHR-2rheometer. Samples were maintained at 23° C. on a Peltier plate.Oscillatory frequency sweeps were performed to probe the materials. Theangular frequency range was 0.1 to 100 rad/s. Oscillatory strain was setat 0.5% to ensure all tests were within the linear range.

The results show that with increasing solid concentration, syntheticfeces possesses higher elastic modulus (G′) and viscous modulus (G″)when the artificial feces change from soft to hard solids with phaseangle decreasing.

Adhesion Measurement

A TA Instruments DHR-2 rheometer was used to measure axial adhesionforce using a probe method. Artificial feces were casted into a PDMSmold (25 mm×25 mm×4 mm) to maintain volume and placed on the lowerPeltier plate kept at 23° C. Sample surfaces (25 mm×25 mm) were boundedto the rheometer upper head. First, the upper head drove the surfacesdownward at 500 μm/s pressing the artificial feces to 1 mm high. Maximumpreloading force was measured in this stage. Secondly the syntheticfeces were left to relax for 5 min. Finally, the rheometer upper headmoved upward at 10 μm/s until full detachment (axial force dropped tozero). Work of debonding, i.e. work done by the upper head in the finalstage, was calculated to characterize the adhesion properties.

Water Consumption Measurement

The open channel flushing system include the following components: awater tank as flow source; pipes; a pump; a valve; a square duct as theflushing open channel; sample mount printed by a 3-D printer.

Bacteria Attachment Experiment of Rainwater

Rainwater was collected from house roof at Park Crest Terrace, StateCollege, Pa., USA. The rainwater was maintained at 4° C. before beingused for experiment. The open channel system was sterilized by alcohol(70%) before the flushing experiment. Solid agar was made by mixingMueller Hinton broth powder (21 g/L) and agar powder (1.5 wt %) into DIwater, and sterilizing for 2 hours. Before the agar solidify, we pouredit onto glass slides (50×75 mm²), and let the agar solidify. Then athick film of solid agar was made. We adhered the agar films ontosurfaces (glass, MR-SLIPS, NP-SLIPS, LESS) flushed by rainwater (1 gpm,10 s), and culture them at 37° C. for 48 hours. As we used glass slides(25×75 mm²) as base substrates, we counted bacteria colony within thearea of 25×25 mm². In each surface, we used at least two differentsamples for the experiments. The error bars are the standard derivationsresulting from counting the bacterial colonies in at least 6 differentareas.

Bacteria Attachment Experiment of E. coli Contaminated Urine

E. coli culture solution was mixed into synthetic urine, with aconcentration of 5×10⁵ cfu/mL. 100 mL of this E. coli contaminated urinewas poured onto the test surfaces. Solid agar was made by mixing MuellerHinton broth powder (21 g/L) and agar powder (1.5 wt %) into DI water,and sterilizing for 2 hours. The agar was poured onto glass slides(50×75 mm²) to solidify. Then a thick film of solid agar was formed. Weadhered the agar films onto surfaces (glass, MR-SLIPS, NP-SLIPS, LESS)flushed by rainwater (1 gpm, 10 s), and culture the samples at 37° C.for 24 hours. As we used glass slides (25×75 mm²) as base substrates, wecounted bacteria colony in area of 25×25 mm². In each surfaces, we usedat least two different samples for the experiments. The error bars arethe standard derivations resulting from counting the bacterial coloniesin at least 6 different areas.

Sliding Angle Measurement

10 μL of bacteria contaminated urine was put onto the test surfaces(glass, MR-SLIPS, NP-SLIPS, LESS). A goniometer was used to measure thesliding angle. We gradually tilted the stage at a speed of 0.03°/s, andtook an image at every second. Then we reviewed the images, anddetermined the tilting angle at which the droplet began to move. Thistilting angle is defined as the sliding angle.

Bacterial Contamination on Different Slippery Surfaces

A series of experiments were conducted to explore bacterialcontamination on different slippery surfaces. First, we sterilizedMR-SLIPS, NP-SLIPS, and LESS before lubrication by wiping them withalcohol (70%) wipes, and then lubricated them with sterilized siliconeoil. Solid agar film was adhered onto these surfaces afterwards forbacterial culture. MR-SLIPS and NP-SLIPS both showed bacterialcontamination; while LESS was free of observable bacterialcontamination. Second, we sterilized MR-SLIPS, NP-SLIPS, and LESS beforelubrication by immersing them into alcohol (70%) for 1 hour. Then solidagar film was adhered onto these surfaces afterwards for bacterialculture. MR-SLIPS had serious bacterial contamination. NP-SLIPS hadnoticeable bacterial contamination. Only LESS was free of bacterialcontamination. The result was shown in FIG. 12 . Third, we focused onsterilizing MR-SLIPS. MR-SLIPS before lubrication were sterilizedseparately by immersing into alcohol (70%) and bleach each for 1 hourrespectively, and then proceed to baking at 150° C. for 10 min. Wenoticed that it was very hard to sterilize the slippery surfaces withroughness. Only bleach would remove the bacterial contaminationcompletely.

Scanning Electron Microscope (SEM)

The SEM images for roughened glass with a conductive 30 nm platinum filmwas taken by Zeiss scanning electron microscope.

The SEM images of bacteria on glass with a conductive 30 nm platinumfilm was taken by Zeiss scanning electron microscope.

Component of Synthetic Feces

The component of synthetic feces used in all experiments is list in thefollowing table. The nutrition percentage is similar to human feces.See, e.g., Rose et al., The Characterization of Feces and Urine: AReview of the Literature to Inform Advanced Treatment Technology.Critical Reviews in Environmental Science and Technology 45, 1827-1879,(2015).

Ingredients % dry mass Nutrition Yeast 32.49 Biomass Psyllium 10.84Fibre Peanut oil 17.31 Fat Miso 10.84 Fibre/Protein/Fat Polyethyleneglycol 12.14 Carbohydrate Inorganic calcium phosphate 10.84 BiomassCellulose 5.53 Carbohydrate

Only the preferred embodiment of the present invention and examples ofits versatility are shown and described in the present disclosure. It isto be understood that the present invention is capable of use in variousother combinations and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein. Thus, for example, those skilled in the art will recognize, orbe able to ascertain, using no more than routine experimentation,numerous equivalents to the specific substances, procedures andarrangements described herein. Such equivalents are considered to bewithin the scope of this invention, and are covered by the followingclaims.

What is claimed is:
 1. A coating on a smooth surface of a substratehaving hydroxyl functional groups thereon and an average roughness Ra ofless than about 1 μm, the coating comprising: a chemical layer graftedon the substrate surface formed from polymerizing a silane or siloxanefrom the hydroxyl functional groups on the surface of the substrate; anda lubricant layer disposed over the chemical layer, the lubricant layerformed from a lubricant.
 2. The coating of claim 1, wherein thelubricant has a viscosity of from about 18 cSt to about 1,000 cSt at 25°C.
 3. The coating of claim 1, wherein the surface has an averageroughness Ra of less than about 100 nm.
 4. The coating of claim 1,wherein the chemical layer grafted on the substrate surface comprises apolydimethylsiloxane grafted on the surface of the substrate.
 5. Thecoating of claim 1, wherein the smooth surface of the substratecomprises silicon.
 6. The coating of claim 1, wherein the smooth surfaceof the substrate comprises a metal, ceramic or glass.
 7. The coating ofclaim 1, wherein the smooth surface comprises the substrate of a medicaldevice.
 8. The coating of claim 1, wherein the smooth surface is asmooth surface of a toilet, urinal or sink.
 9. The coating of claim 1,wherein the lubricant layer comprises a plant oil.
 10. The coating ofclaim 1, wherein the lubricant layer comprises a silicone oil.
 11. Acoating on a smooth surface of a substrate, the coating comprising achemical layer on the smooth surface and a lubricant layer adhered tothe chemical layer; wherein the smooth surface has an average roughness(Ra) of less than 4 μm; wherein the chemical layer is formed bypolymerizing a silane or siloxane or both from hydroxyl functionalgroups on the smooth surface of the substrate to form a chemical layerof graft polymers having ends anchored to the smooth surface; andwherein the lubricant layer is formed from a lubricant having aviscosity of from 1 cSt to about 1,000 cSt at 25° C.
 12. The coating ofclaim 11, wherein the surface has an average roughness Ra of less than 1μm.
 13. The coating of claim 11, wherein the chemical layer has athickness from 1 nm to m and the lubricant layer has a thickness of from1 nm to 1 mm.
 14. A process for preparing a repellant smooth surface,the process comprising: applying a coating composition onto a smoothsurface of a substrate having hydroxyl functional groups thereon,wherein the smooth surface has an average roughness Ra of less than 4 μmand wherein the coating composition includes: (i) a silane or siloxaneor both, (ii) a solvent and (iii) an acid catalyst; anchoring the silaneor siloxane or both from the hydroxyl functional groups on the smoothsurface of the substrate; and applying a lubricant on the chemical layerto adhere and maintain a lubricant layer on the smooth surface and formthe repellant smooth surface.
 15. The process of claim 14, wherein thechemical layer has a thickness from 1 nm to 10 μm and the lubricantlayer has a thickness of from 1 nm to 1 mm.
 16. The process of claim 14,wherein the lubricant comprises a plant oil.
 17. The process of claim14, wherein the lubricant comprises a silicone oil.
 18. The process ofclaim 14, wherein the lubricant has a viscosity of from about 1 cSt toabout 1,000 cSt at 25° C.
 19. The process of claim 14, furthercomprising re-applying the lubricant or applying second, differentlubricant to form the lubricant layer adhered to the chemical layer.